1. Field of the Invention
The present invention relates in general to the fabrication method for a ROM device, and more particularly to a ROM device which uses a shockly diode to form a memory cell and a method of fabricating the same.
2. Description of the Related Art
ROM devices are widely used in mini-computer systems, such as micro-processor systems, to store system data. For example, the BIOS system uses ROM devices. The fabricating processes for a ROM device can take a lot of time and require extensive processing. As a result, customers generally pass programs to be included in their system to a factory that fabricates ROM devices, so that the factory can encode the programs into the ROM devices to complete the products.
Most ROM devices have similar structures, and differ only in the data which is stored during the programming step. Typically, once the integrated circuit (IC) factory receives a programming order from a customer, a mask is produced to perform the programming. This allows production to be finished rapidly, thus reducing manufacturing time and providing better service to the customers. Therefore, ROMs that are post-programmed with the aid of a mask are commonly used in the art.
In general, channel transistors are used as the memory cells in the ROM. In the programming process, specific channel regions of the channel transistors are selectively implanted with impurities to modify the threshold voltages of the transistors. This controls the conductivity of the memory cell. For a detailed description, reference is made to FIGS. 1A through 1C, in which a conventional ROM device is illustrated. FIG. 1A is a top view of a portion of the known ROM device. FIG. 1B is a front view of a portion of the known ROM device. FIG. 1C is a cross-sectional view of the known ROM device.
The conventional ROM device includes a substrate 10, a plurality of bit lines (BL) 11, an oxide layer 12, and a plurality of word lines 13. Substrate 10 comprises, for example, a P-type substrate. The bit lines 11, oxide layer 12, and the word lines 13 are formed on the substrate 10. As shown in FIG. 1A, the region defined by the dotted line is forms a memory cell 14. The data (in binary form such as "0" or "1") is stored in the memory cell by implanting the channel region 16 with impurities.
As shown in FIG. 1C, an N-type dopant, for example, arsenic ions, is doped on the substrate to form bit lines 11, which are arranged with the same distance between each other. Channel region 16 is formed between adjacent bit lines 11. Next, oxide layer 12 is formed on a surface of the bit lines 11 and channel regions 16 using an oxidation process. A conductive layer of, for example, heavily doped polysilicon, is formed over the substrate 10 using a photolithography and etching process to form word lines 13, cross the bit lines 11. Then, channel transistors are formed to complete the partially manufactured conventional ROM.
To program the partially manufactured ROM, a series of continuous programming steps are performed. A mask 15 is formed over the partially completed ROM, leaving the channel region 16 which is to be encoded exposed. Then, a P-type dopant, such as boron, is implanted to form the code. The type of the implanted dopant selected depends on the characteristics of the transistor.
In the conventional ROM described above, the memory cell of the ROM is formed by the channel transistor. The fabricating processes are complicated and the step of code implanting must be formed before the channel transistor is finished. In addition, the area of the MOS device is difficult to minimize. For example, the punch through phenomenon may occur when minimizing the area of the MOS device. Thus, the integrity of the device is limited and it is difficult to improve the integrity.